The present invention relates to a fan-beam antenna which is used in a radar system etc. and in which a level surface beam width is made narrow and a vertical surface beam width is made wide, and further in which dielectric lenses are used together for an antenna in which a vertical surface directivity is restricted by a horn-shaped flare.
In a radar system detecting a target by scanning a directivity antenna over a whole circumference or in a specific sector, an array antenna is provided with a flare such as a slot array antenna in which radiation elements are arranged in a horizontal direction to reduce the horizontal surface beam width and to restrict a vertical surface beam width easily by the horn-shaped flare in a vertical direction.
Proposals such that a gain is secured while restraining an opening of the flare to a practical size by such array antenna with a flare, for instance an S-band radar for shipping, namely such that a vertical surface beam width is made narrow, are shown in JP 60-261204 A and JP 62-171301. It can be considered that these antennas are constituted by projecting several thin dielectric plates in two or three wavelengths to a radiation direction, so that these dielectric plates serve as a waveguide such as a dielectric rod antenna, or it is a dielectric antenna with a small dielectric constant in case of considering an average dielectric constant to a space around the dielectric plate.
On the other hand, it is considered that using a dielectric lens (6) which consists of a single material and is constituted in a convex lens shape as shown in FIG. 6 illustrating an embodiment which is made practicable in a pencil beam antenna, a method for restraining reflection by setting a dielectric lens (7) so as to decrease a dielectric constant in a border surface to a space and to increase the dielectric constant to a center portion of the lens gradually as shown in FIG. 7, or a method for restraining reflection by forming adjustment layers in a dielectric lens (8) by covering a dielectric lens (8a) having a large dielectric constant with a dielectric (8b) having a relatively small dielectric constant (1/the square root) at a thickness that an electric length becomes a quarter wavelength is applied to a fan beam antenna.
In an example disclosed in JP 60-261204 A or JP 62-171301 A, there is a disadvantage such that a size in a propagation direction becomes larger though a vertical size can be restrained in a method for projecting the above mentioned dielectric plate with a few wavelengths. Besides, in the case of using a single material dielectric lens as shown in FIG. 6, it is necessary to consider the reflection due to the dielectric.
It is generally known that a wave impedance z1 in a medium with a relative permeability 1 and a relative dielectric constant εr1 is in the following relationship if a wave impedance in a space with εr0=1 is z0.z1=z0·√{square root over (εs1)}  {circle around (1)}
A coefficient of reflection Γ in a border surface between the medium and the space is shown in the following expression {circle around (2)}.
                    Γ        =                                            z1              -              z0                                      z1              +              z0                                =                                                    ɛ                ⁢                                                                  ⁢                s1                            -              1                                                      ɛ                ⁢                                                                  ⁢                s1                            +              1                                                          2        ⁢        ◯            
Furthermore, a voltage standing wave ratio (VSWR) in a border surface between the medium and the space can be shown in the following expression {circle around (3)}.
                    VSWR        =                                            1              +              Γ                                      1              -              Γ                                =                      ɛ            ⁢                                                  ⁢            S1                                              3        ⁢        ◯            
According to the expression {circle around (3)}, for instance, if it is desired that VSWR in a border surface between a dielectric and a space is restrained to 1.2, the relative dielectric constant is 1.2. Besides, in the case that border surfaces are two as shown in FIG. 6, two reflection are composed. When considering the worst value, it is necessary to halve each coefficient of reflection Γ, so that the relative dielectric constant is approximately 1.1 when it is looked for by using the expression {circle around (2)}. As a result, it is found that it is necessary to use a material with a much lower dielectric constant. Thus, it can be supposed easily that a thickness of the lens becomes larger, and further problems arise in a forming or a means for securing.
Moreover, as shown in FIGS. 7 and 8, if a dielectric constant in a center portion can be larger, a thickness of the lens can be thinner, but a method for manufacturing compound materials is difficult, so that these methods are rarely used in a fan-beam antenna.